1. Field of the Invention
This invention relates to a metal gasket interposed between the opposed surfaces of parts of a multicylinder engine, and adapted to seal a clearance between these parts.
2. Description of the Prior Art
In order to seal a clearance between a cylinder head and a cylinder block of an engine, a gasket made of a metallic material has been used. In a recent engine, the increasing of an output and the reduction of the weight have been demanded, and, in order to meet the demand, a cylinder head and a cylinder block, or a cylinder head alone tends to be made of an aluminum material of a low specific gravity instead of conventionally used steel and casting of a high specific gravity.
An aluminum cylinder head and an aluminum cylinder block, structural members of an engine have a small weight but a low rigidity, so that a relative displacement tends to increase between these structural members during an operation of the engine. Various kinds of metal gaskets are inserted between the two structural members so as to seal a clearance between the opposed fixing surfaces thereof. Especially, a distance between cylinder bores has become shorter in accordance with a decrease in the weight of an engine, and a distance between combustion chamber bore holes of a metal gasket has also become shorter in accordance with such narrow regions between the cylinder bores. Therefore, it is demanded that a predetermined level of sealing performance of the metal gasket be secured by forming beads on the corresponding narrow regions of the metal gasket.
A metal gasket is provided with beads on the portions thereof which are close to the circumferences of through holes thereof which correspond to cylinder bores, i.e. combustion chambers and passages for water and oil, in such a manner that, when a cylinder head and a cylinder block are fixed to each other by tightening the same by bolts, the beads form elastic annular contact portions with respect to the opposed surfaces of these engine-forming members and seal a clearance therebetween.
In an engine provided with a cylinder head gasket, a clearance between a cylinder head and a cylinder block increases and decreases repeatedly during a combustion cycle of the engine, and stress, i.e. mechanical stress and thermal stress are exerted repeatedly on the metal gasket. The load fluctuation stress on an engine occurs greatly in the portions of a cylinder block and a cylinder head which have the lowest rigidity, and, especially, bead portions between combustion bore holes receive an extremely heavy load. Consequently, permanent set in fatigue and cracks occur in the beads, and the sealing performance of the metal gasket is deteriorated.
The conventional cylinder head gaskets include the cylinder head gasket 1 disclosed in Japanese Patent Laid-Open No. 233105/1996. This cylinder head gasket 1 is applied to an open deck, and comprises a metal plate of a larger thickness forming the portions of the gasket 1 which are on the inner side of a water jacket 12 and around the circumferences of cylinder bore holes 2, and a metal plate of a smaller thickness forming the portions of the gasket 1 which are other than these circumferential portions 14 of the cylinder bore holes 2 and the portions 15 to be superposed on the thicker metal plate, these two metal plates 14-15 being superposed on each other, the superposed portions 15a being then welded.
The cylinder head gasket 1 disclosed in this publication is formed of divided metal plates 14-15 constituting a cylinder bore hole 2 surrounding portion 14 and the remaining portion 15 respectively, and the two metal plates 14-15 are welded at a water hole 12 surrounding portion. Therefore, the portions 14-15 to be welded of the metal plates are superposed on each other to cause the thickness of the welded portion 15 a to increase extremely as compared with that of the remaining portion 15. This cylinder head gasket 1 can be applied to an open deck in which a water groove 12 is provided in a water jacket in the opposed surfaces of the cylinder block 11 and cylinder head 13 but it is impossible to insert this gasket 1 directly between the opposed surfaces of other type of cylinder head 13 and cylinder block 11 and bring the same into contact with these opposed surfaces.
There is another metal cylinder head gasket 1 disclosed in Japanese Utility Model Laid-Open No. 39467/1992. This metal cylinder head gasket 1 comprises an elastic metal plate provided with cylinder bore holes 2.2A.about.2D, half beads 5.5A.5B extending along the circumferences of these cylinder bore holes, and beads 4.4A.about.4D extending along the half beads 5.5A.5B, and wire rings 6.6A.6B the flat surfaces of which are fixed to annular surfaces formed between the beads 4.4A.about.4D and half beads 5.5A.5B on the elastic metal plate. This metal cylinder head gasket 1 is formed with flat wire rings 6.6A.6B provided around the cylinder bore holes 2.2A.about.2D Since one wire ring 6.6A.6B is provided around each cylinder bore hole 2.2A.about.2D, the portions between adjacent cylinder bore holes 2.2A.about.2D are necessarily set to a width equal to the sum of that of a bead 4 and that of two wire rings 6. However, the weight of a recent engine has further been reduced, and a distance between cylinder bores has become shorter, so that it is difficult to provide two wire rings 6 between adjacent cylinder bore holes 2. Moreover, fixing wire rings on an elastic metal plate of a gasket causes the stability of the gasket to lower. Since the wire rings 6 require to have a predetermined thickness with the width of the regions between the cylinder bore holes 2 set small, this structure has the difficulty in using a fixing means, such as a welding means for securing the wire rings to the elastic metal plate.
When impressions occur in the surfaces of a cylinder head and a cylinder block 11 or when irregularity occurs between the opposed surfaces thereof, a large strain occurs in, especially, the regions between combustion chamber bores, and a high-temperatures, high-pressure combustion gas enters a space between the portions of the opposed fixing surfaces which have a large strain to cause the beads 4.4A.about.4D on the elastic metal plate to be corroded and polluted, permanent set in fatigue and cracks to occur in the beads 4.4A.about.4D, and the sealing performance of the gasket 1 to lower.